1. Field of the Invention
The present invention relates to processes for producing silanes having an ester group with a silicon atom in the alcohol-derived portion of the ester.
2. Description of the Related Art
Ester-functional silanes having a silicon atom in the alcohol-derived portion of the ester can be represented by the formula I, which also illustrates which portion of the ester is referred to as the “alcohol-derived portion” and which is referred to as the “carboxylic acid-derived portion”:R1—C(═O)—O—R2—SiR33 acid-derived portion←|→alcohol-derived portion  Formula I
When none of the R3 radicals is hydrolyzable, the esters are obtainable by reacting an alcohol of the structure HO—R2—SiR33 with a carboxylic acid of the structure R1—C(═O)—OH or with derivatives thereof (e.g., ester, acid anhydride, acid azide; general structure: R2—C(═O)—Y; —Y=—OH, —O-alkyl, —O-aryl, —O-acyl, —N3, —Cl etc):R1—C(═O)—Y+HO—R2—SiR33→R1—C(═O)—O—R2—SiR33+H—Y  Reaction 1H—Y can be trapped with an auxiliary base, if necessary. When at least one of the R3 radicals is hydrolyzable, an alcohol of the structure HO—R2—SiR33 will condense with itself by eliminating one H—R3 equivalent to form a sila-oxacycle or its oligomer. The reaction pathway depicted in reaction 1 is therefore regrettably not applicable to such silanes.
Alternatively, some of these ester-functional silanes are obtainable by attaching an ester of the structure R1—C(═O)—O—CH═CH2 or of the structure R1—C(═O)—O—R′—CH═CH2 by hydrosilylation to a silane which has at least one Si—H bond (reaction 2). However, this method is restricted to ester-functional silanes with at least two carbon atoms between the silicon atom and the ester function; hydrosilylation frequently shows poor regioselectivity at the double bond; and hydrosilylation catalysts frequently also catalyze cleavage of the C(O)O—C bond as a simultaneous reaction, especially when allyl esters are involved.
The third usable type of reaction for preparing such silanes involves a salt of a carboxylic acid, i.e., a carboxylate, being reacted with a silane having an Si-attached hydrocarbon group with a leaving group “X”:[R1—C(═O)—O]a[Ma+]+aX—R2—SiR33-->aR1—C(═O)—O—R2—SiR33+[Ma+]X−a  Reaction 3
This reaction pathway does not have the disadvantages of the first two pathways. For instance, it is also applicable to silanes having hydrolyzable radicals R3 or only one carbon atom in R2. As will be apparent, however, the salt [Ma+]X−a is formed along this route. An aqueous workup to remove the salt is only possible if the product is not hydrolysis-sensitive, which however will be generally the case if the silanes thus obtained are to be sufficiently reactive at one or more of the R3 groups to permit using the silanes for producing organofunctional silicones or for moist crosslinking. Nonaqueous removal of the salt requires a centrifugation or filtration step. Such methods are described for example in WO 2007 063 011 or in Monatshefte für Chemie, 2003, volume 134, pp. 1081-1092. The salt has to be washed at some cost and inconvenience, yet product residues frequently remain in the salt despite washing and are lost in this way. After filtration, the filtrate can be subjected to a fractional distillation. If the filtration step were omitted and the distillation were to be carried out directly in the presence of the salt, the salt would increasingly dry out in the distillation pot during the distillation and become encrusted therein, the commixing and hence the separation efficiency of the distillation would become increasingly worse, frequently leading to local overheating and hence to the risk of exothermic decomposition reactions being triggered. The salt bakes onto the distillation pot during the distillation and is very difficult to remove thereafter. Furthermore, unconverted carboxylate can trigger undesired side-reactions in a distillation without a prior filtration if R1, R2 or R3 contain sensitive groups. If a solvent were to be used in the reaction or to wash the salt, the solvent first has to be time-consumingly distilled off before the product can be recovered. It would be desirable to have a process that permits distillation directly in the presence of the salt without any filtration step, ideally without product being lost as the result of silane of formula I adhering to the salt (as is generally the case with a filtration) or that avoids the laborious distillative removal of the solvent prior to the actual product distillation, or ideally both.